Relax bound on blanket impl for Source

[ybh1998]

The current definition in this library is not friendly for dynamic use like dyn polling::Source, because the blanket implementation of Source for &T (instead of T). Consider the following code:

polling/src/lib.rs

Lines 539 to 543 in ca52490

	impl<T: AsRawFd> Source for &T {
	fn raw(&self) -> RawFd {
	self.as_raw_fd()
	}
	}

The current implementation is effectively accepting a double reference because there are references in both the receiver type and &T, and it does not align with other traits that we might also be interested to have (e.g., Read, Write). Consider the following attempt to have a dynamic Source which is also Readable and Writeable:

pub trait ReadWriteSource: Read + Write + polling::Source {}
impl<T: Read + Write + polling::Source> ReadWriteSource {}

Unfortunately, (for example) TcpStream does not implement ReadWriteSource because it does not implement Source.

With the proposed changes, types TcpStream will properly implement ReadWriteSource. A platform-agnostic return type RawSource for Source::raw is also added to help if the users want to implement polling::Source by themselves.

[notgull]

This PR has been displaced by the merge of #123 changing the concept of the Source trait. Feel free to re-open if this is updated for the new codebase.

Thanks anyways!